Vent plug for electrolytic capacitor



Oct' 9 1956 A. M. Gr-:oRGlEv ET AL 2,766,408

VENT PLUG FOR ELECTROLYTIC CAPACIATOR Filed Dec. 6, 1954 I9 um, mgl" l5@n '6 2 F/g 2 IN1/EN ToRs Alexander M. Georgiev James L. ancock BY H@QM- www Their Attorney United States Patent O VENT PLUG FORELECTROLYTIC CAPACITOR Alexander M. Georgiev, Clearfield, Pa., and JamesL. Hancock, Fairborn, Ohio, assignors to General Motors Corporation,Detroit, Mich., a corporation of Delaware Application December 6, 1954,Serial No. 473,113 6 Claims. (Cl. 317-230) This invention relates toelectrolytic condensers or capacitors, and more particularly to a ventconstruction or safety device provided in the wall of the capacitor torelieve excess gas developed Within the capacitor unit during itsoperation.

It is well known that during the operation of electrolytic capacitorsthat -substantial quantities of gas may be developed within thecapacitor unit due to various causes such as high alternating current ofan AC operating voltage or accidental polarity reversals in theso-called polarized capacitors due to a rapid reforming of thedielectric or blocking film after prolonged periods of idleness, heavyoverload conditions and many other reasons well known to those skilledin the art.

It has therefore become a general practice to protect capacitor unitsagainst explosion resulting from high pressure gas within the capacitor,the gas being retained within the capacitor as a result of completesealing of the capacitor against iniltration of moisture and otheragents having a destructive effect on the capacitor. The capacitor unitsare usually hermetically sealed.

Safety or venting devices have therefore been used on capacitors toprovide for escape of high pressure gas from the capacitor unit when thepressure of the gas rises to a dangerously high value.

The highpressure gas developed within a capacitor is also accomplishedby a substantial rise in temperature of the capacitor above normaloperating temperature, the temperature and the gas pressure risingconcurrently.

It is therefore an object of this invention to provide an improved ventor safety device that is, particularly suitable for electrolyticcapacitors, to be responsive only to concurrently existing heat andpressure conditions before providing for venting of the capacitor.

It is another object of the invention to provide a vent or safety plugin a wall of a capacitor that is formed of a thermoplastic material thatis semi-rigid at all nor mal operating temperatures of the capacitor toeffect thereby a pressure sealing engagement in an opening provided inthe wall of the capacitor, the thermoplastic material being heatsoftenable increasingly upon a progressive rise in temperature of thecapacitor with concurrent pressure rise internally of the capacitor sothat at a predetermined temperature the heat softenable plug will be ofsucient softness and resilience to permit the internal pressure in thecapacitor existing at that temperature to force the heat softened andnow resilient plug from the opening of the wall of the capacitor forescape of the gas from the capacitor.

lt is another object of the invention to provide an proved vent orsafety device for an electrolytic capacitor or Vcondenser wherein eitherpressure or temperature conditions alone will not effect an injection ofthe safety vent plug from an opening in the wall of a capacitor, but atnormal operating temperatures the vent plug or safety device will remainsemi-rigid to prevent accidental displacement of the vent plug eventhough internal pressure in the capacitor may rise temporarily abovenor- 'ice mal operating conditions, and so long as the internal pressurein the capacitor remains at a normal low level, some rise in temperatureof the capacitor will not effect a failure of the vent.

It is another object of the invention to provide a vent plug or safetydevice for an electrolytic capacitor in accordance with the foregoingobjects wherein the plug is made of a semi-rigid heat softenablethermoplastic material that is preferably suiiciently heat softenable atabout 200 F. to provide for ejection of a plug from an opening in theWall of a capacitor when pressure rises to a value in the neighborhoodof 10() p. s. i. Such heat softenable thermoplastic materials areavailable commercially on the market, for example, polyethylene,polystyrene, nylon, polyvinyl chloride, cellulose acetate, butyrate, andmethyl methacrylate.

Further objects and advantages of the present invention will be apparentfrom the following description, reference being had to the accompanyingdrawings wherein a preferred form of the invention is clearly shown.

In the drawings:

Figure l is a cross sectional view of an electrolytic capacitor orcondenser incorporating the subject of this invention.

Figure 2 is a partial cross sectional view illustrating a modified formof the vent plug of the invention.

Figure 3 is a further modied form of the vent plug of the invention.

In this invention the capacitor or condenser comprises a metal can 10 inwhich there is placed a conventional electrolytic condenser or capacitor11 which may consist of two electrode strips of aluminum or other iilmforming metal separated by strips of a dielectric paper, the superposedelectrode and spacer strips being wound into a capacitor or condenserunit with tabs i2 and 13 extending from separate electrodes. Aninsulating shell 14 of dielectric material is placed between the metalcan and the capacitor roll 11.

The terminal tabs 12 and 13 are suitably connected to the terminal pins15 and 16 that are mounted in the cover 17. The cover 17 comprises arigid body that engages the shoulder 18 of the can 19, the outer end 19of the can 1t) being formed over a resilient washer 2t) to hermeticallyseal the can 1t).

To provide for venting of gases evolved within the container 10 duringsome abnormal operation of the capacitor, a vent plug or safety device2S is provided in the cover 17 of the capacitor.

The cover 17 is provided with a tapered opening 26 that receives thevent plug 25.

The vent plug 25 is in the form of a cone and conforms generally to thetaper of the wall of the opening 26. The plug 25 thus comprises a hollowcone shaped member having a thin wall 27 that engages the wall of theopening 26. The outer end 2? of the plug is closed while the hollowinterior 29 of the plug 25 is open to the gases created within thecontainer 1t).

The plug 25 is provided with an outwardly extending hanged portion 30 onthe end of the plug that is positioned within the container. The flangeportion 3' is adapted to engage the inner face 31 of the cover 17.

The vent plug 25 is made of a thermoplastic material that is semi-rigidat all normal ambient or room temperatures and somewhat elevatedthereabove, but which material is heat softenable as the temperaturebecomes progressively higher, the softening of the thermoplasticmaterial causing it to become resilient and ilowable through the opening26 when high pressure also exists internally` of the container 10.

There is available on the commercial market any number of thermoplasticmaterials that are particularly suited to use as vent plug materials,the primary `factor being that they shall be suciently heat softenablein a range from ISO-200 F. that they are sufficiently soft to allow theplug 25 to be blown outwardly from the opening 26 when a pressure offrom 80-100 p. s. i. exists within the container lll. SuitablethermoplasticsV that are available are: polyethylene, polystyrene,nylon, polyvinyl chloride, cellulose acetate butyrate, methylmethacrylate. These thermoplastic materials are normally semi-rigidunder temperature conditions that' might be termed non mal ambient orroom temperature, that is below about 1.30", or thereabouts. Astemperatures increase substantially above what might be termed normalambient temperature conditions the material still remains in a semirigid condition but in the range of about l`75200 F.

there is a definite softening of the material fromth'e semiv rigidcondition. The materials are not yet what might Y be termed flowablefrom the standpoint of compression or injection molding of thematerials, yet they have lost a substantial portion of theirrigidityiand have in elfect be-A come soft and resilient.

When theA thermoplastic safety vent plug 25 is in the softened conditionjust referred to, high pressure within the container lt) can force theplug 25 outwardly from the opening 26 and thereby vent the high pressuregases from within the container lil.

It is an anomaly that high pressure conditions within the capacitorcontainer lil coexist only with high temperature conditions of thecapacitor. Thus heat softening of the vent plug 25 occurs at the sametime thatV high pressure conditions exist within the capacitorcontainer.

The heat softening point of the vent plug 25 can thus be selectedagainst the maximum pressure rise desired in the capacitor container toprovide for suflicient softening and resilience of the vent plug 25 atthe selected maximum temperature desired in the container to effect ablow out of the vent plug from the opening 26 when; the predeterminedtemperature and coexisting pressure conditions are established in thecapacitor.

Also, the point at which the blow out can occur under selected pressureconditions can be Varied by a change inv the angle of the cone shape ofthe opening 26 and the vent plug 25, a gradual increasing of thediameter of the base of the cone relative to the apex generallyrequiring higher pressure and high temperature conditions to exist inthe capacitor before the thermoplastic vent plug 25 will be softenedsufficiently to provide for a blow out of the plug Vat the coexistingpressure and temperature conditions.

Also, the cone shape of the opcning26 and the vent plug 25, asillustrated in Figure l, provides for a pressure sealing of the ventplug 25 in the opening 26 as the vent plug gradually softens until suchtime as the pressure rises to the predetermined value and effects acomplete ejection of the Vent plug from the opening. 26. The ther-Kmoplastic vent plug thus provides an accurately controlled point ofcoexisting temperature pressure values at which the vent plug will beejected from the opening 26. If either the temperature or the pressure*is of lower value than that required for the ejection under thepre-established coexistent pressure temperature condition, the vent plugwill not be ejected from the opening 26. If either of these pressure ortemperature conditions of lower value should independently exist in thecapacitor there is the major probability that no serious damage isoccurring to the capacitor and therefore there is no particular reasonfor causing the vent plug to effect a ventingof the capacitor. Suchconditions might exist for example in the effect of an abnormally highroom ambient temperature without coexisting high pressure in thecapacitor. Under this condition there is no gas evolutionin thecapacitor that represents a danger of` destruction of the capacitor andeven though the vent plug 25 may be materially softened by theabnormally'hgh room temperature, there isno. reason for venting ofthecapacitor; because` pressure conditions are not.sufficientlyhighV as toeiectdestnlction ofthe capacifv tor. Obviously a quick short circuit inthe capacitor that might be self healing could also occur to provide atemporary high pressure condition without resultant heat of thecapacitor. Here again it would not be desirable for the capacitor tovent.

In Figure 2 there is illustrated a slightly modiiied form of safety vent4t) that is made of thermoplastic material as heretofore described; Inthis modificationV the vent plug 40 is made in a true cylinder shape andwould be ejected more readily from the opening 41 inthe cover 17a thanthe form of the vent plug 25 illustratedY in Figure l.

In Figure 3 there is illustrated' still another form of thermoplasticvent plugy 50 in which the cone shape of the vent plug and the opening5l in the cover 52 is just reverse of that illustrated inr Figure l. lnthe arrangement shown in Figure 3 the vent plug would be ejected morereadily from the opening 5l than that illustrated in either Figure l or2.

Hence it can be seen that modiication of the longitudinal contour of thevent plug can be made to provide a variable factor at which coexistingeffective temperature. and pressure conditions will eject the vent plugfrom the capacitor to vent the capacitor.

While the form of embodiment of the invention. as. herein disclosedconstitutes a preferred form, it -is to be understood that other formsmight be: adopted as may come within the scope of the claims whichfollow.

What is claimed is as follows.

l. In an electrolytic condenser, the combination. of av gas imperviouscontainer adapted to enclose a condenser unit having an electrolyte, awall of said' container having an opening therein, and a thin walledresiliently semirigid heat softenable hollow plug having one closed endand a generally radially outwardly extending peripheral flange at theopposite end, said plug being disposed in said opening sealing the sameand projecting from each of opposite ends of said opening with saidclosed end forming a bulbous protrusion at one end of said opening andlsaidv flange engaging said container wall at the opposite end of saidopening, said plug being constituted to4 maintain a resilient semi-rigidcondition under all normal operating'V temperatures in resilientpressure sealingengagement with the wall forming the opening to retainthe container sealed and softenable progressively increasingly withprogressive rise in temperature only above normal operating temperaturesto effect an increase in resilience. of said. plug with consequentphysical deformation of the plug on rise in pressure in the container toeect bodily ejection ofthe plug from the opening at predeterminedconcurrently effective pressure and temperature conditions above normalpressure and temperature conditions.

2. A combination in an electrolytic condenser structure in accordancewith claim l in which the said plug has a wall thickness in that portionof the plugV engaging the wall forming the opening that is peripherallyuniform.

3. A combination in an electrolytic condenser structure in accordancewith claim l in which the said plug has a wall thickness in that portionof the plugengaging the wall forming the opening that is peripherallyuniform and in which the end of the plug withinv the container has theoutwardly extending peripheral flange to engager-the face of the wall inthe container in whichV the opening is formed.

4. A combination in an electrolytic condenser structure in accordancewith claim l in which the saidopening has the wall thereof tapered andlsaid plug is correspondingly tapered to lit in said opening.

5. A combination in an electrolytic condenser structure in accordancewith claim 1 in which; the Said opening-hasY a cone shape and said plughas a cone shape, to twithin said opening.

6. A combination in an electrolyticcondenser structure` v in accordancewith claim 1 in which. the. said. openingis conci shaped and said plugis cone shaped to'it within said opening with the plug wall being ofuniform thickness in that portion thereof engaging the wall forming theopening with the end of the plug within the container having theoutwardly extending ange to engage the face of the wall within thecontainer in which the opening is 5 formed.

References Cited inthe le of this patent UNITED STATES PATENTS MenschikDec. 26, 1950

